Question: Find the distance between the point ${(1, -5)}$ and the line $\enspace {y = -x + 4}\thinspace$. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$
Answer: First, find the equation of the perpendicular line that passes through ${(1, -5)}$ The slope of the blue line is ${-1}$ , and its negative reciprocal is ${1}$ Thus, the equation of our perpendicular line will be of the form $\enspace {y = x + b}\thinspace$ We can plug our point, ${(1, -5)}$ , into this equation to solve for ${b}$ , the y-intercept. $-5 = 1 + {b}$ $-5 - 1 = {b} = -6$ The equation of the perpendicular line is $\enspace {y = x - 6}\thinspace$ We can see from the graph (or by setting the equations equal to one another) that the two lines intersect at the point ${(5, -1)}$ . Thus, the distance we're looking for is the distance between the two red points. The distance formula tells us that the distance between two points is equal to: $\sqrt{( x_{1} - x_{2} )^2 + ( y_{1} - y_{2} )^2}$ Plugging in our points ${(1, -5)}$ and ${(5, -1)}$ gives us: $\sqrt{( {1} - {5} )^2 + ( {-5} - {-1} )^2}$ $= \sqrt{( -4 )^2 + ( -4 )^2} = \sqrt{32} = 4\sqrt{2}$ The distance between the point ${(1, -5)}$ and the line $\thinspace {y = -x + 4}\enspace$ is $\thinspace4\sqrt{2}$.